A long standing problem in the art of manufacturing integrated circuits is that of completing a process step and not knowing whether the process step completed successfully. This is also referred to as endpoint detection. If the step did not complete successfully, and the processing of the integrated circuit continues, then it is likely that at the end of the manufacturing process the circuit will not work as designed. Continued processing after a failed process step results in wasting the costs of processing after the failed step.
An in-situ chemical-mechanical polishing process monitor apparatus for monitoring a polishing process during polishing of a semiconductor wafer in a polishing machine is disclosed in U.S. Pat. No. 5,433,651. The polishing machine has a rotatable polishing table, and a window embedded within the polishing table. The window enables in-situ viewing of a polishing surface of the semiconductor wafer. A reflectance measurement device is coupled to the window on the underside of the polishing table for measuring a reflected optical signal from the semiconductor wafer. The reflectance measurement device processes the reflected optical signal which is representative of an in-situ reflectance, wherein a prescribed change in the in-situ reflectance corresponds to a prescribed condition of the polishing process. A drawback of this approach is that as technology nodes continue to shrink, the reflectance meets is limit for the dependency of a previous film stack on the semiconductor wafer. In other words, the optical signal from the reflectance measurement device travels through a very thin film so that it is not properly reflected back to the reflective measurement device.
As an alternative to a reflective measurement device used for endpoint detection that relies on a reflected optical signal, measured surfaced voltages may be used. U.S. Pat. No. 6,844,205 discloses a system that detects the clearing of a dielectric at a plurality of contact sites by measuring the surface voltage of the dielectric, and comparing the surface voltage to a reference voltage set to a value that relates to the cleared contact sites. A non-contact probe measures surface voltage of the semiconductor wafer during removal of the dielectric. The non-contact probe may be a Kelvin probe, for example.
Even in view of the advances made in determining endpoint detection when processing semiconductor wafers, there is still a need to improve how endpoint detection is determined. This is particularly so as semiconductor processes continue to achieve smaller line widths to create semiconductor wafers with greater capacity.